1. Field of the Invention
The present invention relates to a semiconductor device and its production method, and more specifically, to a semiconductor device containing a compound semiconductor transistor having a gate electrode, such as a T-shaped gate electrode, provided with an overhang which widely expands at an upper side thereof, and its production method.
2. Description of the Prior Art
A field effect transistor using a compound semiconductor is widely used as a micro wave device because of its superior high frequency characteristics. Especially owing to prevalence of mobile phones and satellite broadcasting, it has become popular to incorporate a GaAsMESFET, a HEMT and the like comprising a compound semiconductor into an MMIC (monolithic microwave IC), wherein stabilization of a threshold voltage of a transistor and control accuracy of the threshold voltage are important.
As shown in FIG. 1, for example, the GaAsMESFET has an active semiconductor layer 102 containing some impurities which are formed over a semi-insulating compound semiconductor substrate 101, a gate electrode 103 which is formed on the active semiconductor layer 102, and a source electrode 106 and a drain electrode 107 which are formed on the active semiconductor layer 102 at each side of the gate electrode 103 through a contact layer 105. The gate electrode 103, the active semiconductor layer 102, the contact layer 105 and the like are covered with a protection layer 108 built using a material such as silicon oxide and silicon nitride.
In the field effect transistor mentioned above which uses a compound semiconductor substrate, the gate electrode 103 and the active semiconductor layer 102 are respectively stressed and tend to generate electric polarization in the compound semiconductor layer, whereby polarized charge is developed across a semiconductor crystal to cause a piezoelectric effect. If the polarized charge presents, a threshold of a gate voltage becomes instable.
The shorter the length of the gate electrode 103 in a channel length direction (that is, a gate length), the larger the variation of the threshold voltage according to the piezoelectric effect.
In order to stabilize the threshold voltage, uniformity of the quality of the protection film 108 which covers the gate electrode 103 and a stress control of the protection film 108 become necessary.
For example, Japanese Patent Laid-Open No. 95573/1986 describes that impurities are implanted into a gate electrode for controlling a stress from the gate electrode per se. Further, Japanese Patent Laid-Open No. 20629/1989 and Japanese Patent Laid-Open No. 247618/1990 describe that a stress from a gate electrode is canceled utilizing a stress from an insulating film which covers the gate electrode, by which the stress from the gate electrode can be controlled by changing a thickness of the insulating film. In addition, the stress from the gate electrode made only of tungsten silicide (WSi) can be controlled to some extent by changing sputtering conditions for WSi growth.
Still further, Japanese Patent Laid-Open No. 282841/1992 describes that a stress is adjusted by covering a semi-conductor layer at a side of a gate electrode with an insulating film and then exposing the insulating film to oxygen plasma or nitrogen plasma.
In the patent specifications mentioned above, deviation of the threshold voltage caused by a piezoelectric effect is adjusted by covering a gate electrode having a square cross section which is formed by self-align techniques or lift-off techniques, however, these techniques described above can not be applied as they are to the gate electrode which has a simplified T-shaped cross section (hereinafter referred to as a T-shaped gate electrode).
In the T-shaped gate electrode, its narrow bottom surface forms a Schottky contact with a substrate layer. This bottom surface is made small as the gate length is decreased. On the other hand, the upper side of the T-shaped gate electrode widely expands to form an overhang-like extension to the lower side thereof for lowering the resistance, and a low resistance layer made of the other material is often stacked thereon.
In the T-shaped gate electrode which includes such an overhang as mentioned above, a stress cannot be stably controlled since the stresses applied to the upper and lower parts of the electrode are not uniform when a single insulating film is used, and as the result, controlling a threshold voltage becomes difficult.